Superconducting machines usually comprise superconducting coils which must be reliably cooled, at least during the operation of the machine. Metal oxide superconductive materials with critical temperatures Tc of over 77 K have been known since 1987. These materials are also known as High Tc Superconductive (or HTS) materials and, in principle, facilitate a cooling technique using liquid nitrogen (LN2). Machines which include coils with HTS may thus be cooled and/or operated, for example, with liquid nitrogen (LN2) or with liquid neon (LNe).
Refrigeration units in the form of so-called cryocoolers with closed compressed helium gas circuits are preferred for the cooling of windings with HTS material. Such cryocoolers are in particular of the Gifford-McMahon or Stirling type, or are designed as so-called pulse tube refrigerators. They have the advantage that their refrigerating capacity is available almost at the touch of a button and the use of cryogenic fluids is avoided. When such refrigeration devices are used, the superconducting winding is indirectly cooled e.g. by heat conduction to a cold head of a refrigerator (cf. e.g. “Proc. 16th Int. Cryog. Engng. Conf. (ICEC 16)”, Kitakyushu, J P, 20.-24 May 1996, Verlag Elsevier Science, 1997, pages 1109 to 1129).
A refrigeration technique, such as the one described e.g. in DE 103 21 463 A1, can be used for cooling the rotors of electrical machines. The rotor contains a rotating winding comprising HTS conductors, which are located in a thermally conductive winding mount. This winding mount is equipped with a central, axially extending cylindrical cavity, to which are connected pipe sections that lead laterally out of the winding mount. These pipe sections lead to a condenser chamber of a refrigeration unit, which condenser chamber is located in a geodetically higher position, and, together with this condenser chamber and the central rotor cavity, form a closed one-pipe system. This pipe system contains a refrigerant or cooling fluid, which circulates using a so-called thermosiphon effect. Cooling fluid condensate in the condenser chamber is thus transported via the pipe sections into the central cavity, where it absorbs heat and evaporates due to the thermal coupling to the winding mount and thus to the HTS winding. The evaporated cooling fluid then flows, via the same pipes, back into the condenser chamber, where it is re-condensed. The refrigerating capacity required for this purpose is provided by a refrigerating machine, the cold head of which is thermally coupled to the condenser chamber.
This return flow of the refrigerant is driven by a slight overpressure into the central cavity, which acts as an evaporator part, to the parts of the refrigerating machine which act as a condenser unit. This differential pressure, which is produced by the creation of gas in the evaporator and by the liquefaction in the condenser chamber, therefore leads to the desired refrigerant return flow. Corresponding refrigerant flows are known in principle from so-called heat pipes.
Therefore, among the known machines with thermosiphon cooling by means of a suitable refrigeration unit, the liquid refrigerant is transported using only the force of gravity, so that no further pumping systems are required. This requires a refrigeration unit or a condenser chamber, which must essentially be located in a geodetically higher position than the machine and/or the winding mount. The disadvantages associated with this are particularly prevalent where there are spatial limitations around the machine and the refrigeration unit structure. Thus, e.g. in a machine with vertically arranged machine axis, an object driven by the machine, e.g. a motor, may be disposed above the machine. The machine is installed in its surrounding area such that there is no free space available in the plane of the machine. The geodetically higher position is occupied by the object being driven, and a arrangement of the refrigeration unit geodetically higher is not possible in this situation. Even in complex applications, such as e.g. for railcars on railroads, the construction height of the railcar may be restricted e.g. due to regulations relating to the height of overhead lines and/or tunnels. If the machine dimensions are within the range allowed by the height regulations, it may still not be possible for the refrigeration unit to be arranged in a geodetically higher position than the machine, even if the machine axle is positioned horizontally.
A further case in which problems may occur with a purely gravity-driven refrigerant flow is on ships or offshore installations. If a machine installation as described above is used on ships or offshore installations, then static imbalances, known as “trim”, of e.g. up to ±5° and/or dynamic imbalances of e.g. up to ±7.5° longitudinally, are frequently to be expected. In order to obtain approval for use on vessels from a classification body, the cooling system of such a machine installation must consequently guarantee reliable refrigeration on board a seagoing vessel even under these conditions. If the said imbalances of the machine are to be tolerated, then there is a risk that an area of the pipe sections between the central rotor cavity and the refrigeration unit will come to be geodetically lower than the central rotor cavity. As a result, the refrigerant will not be able to reach the rotor cavity to be cooled using the force of gravity. The cooling of the machine and therefore its operation would therefore no longer be guaranteed.
In order to guarantee reliable cooling even in the event of machinery imbalances, it is possible for the machine to be arranged at an angle to the horizontal, so that, even assuming maximum trim positions or oscillation amplitudes in the thermosiphon pipe system, an inclination toward the rotor cavity is always present. However, it is precisely in shipbuilding that such an inclined arrangement is undesirable, especially with greater machine lengths, because of the large space requirement that would be necessitated. Alternately, instead of a one-pipe system for circulating refrigerant between a condenser chamber and the evaporator chamber, in which the liquid and the gaseous refrigerant flow to and from the condenser chamber through the same pipe sections, a two-pipe system may be used. This system uses the thermosiphon effect as described e.g. in WO 00/13296 A. However, an additional pipe for the gaseous refrigerant must be provided in the area of the hollow rotor shaft. The condenser chamber must be geodetically positioned in relation to the evaporator chamber at a height that is sufficient to guarantee a reliable flow of cooling fluid by force of gravity from the condenser chamber into the evaporator chamber. This requires installation space, the availability of which may be limited, e.g. on ships.
A further alternative is offered by the use of a mechanical pump and/or mechanical valves. The refrigerant may be forced through by a pumping system. However, this would necessitate a considerable amount of equipment, particularly if the refrigerant has a temperature of 25 to 30 K, for example. Circulating pumps of this kind cause considerable losses and are scarcely able to fulfill the service life requirements of shipbuilding, which involve long maintenance intervals.